This invention relates to the field of liquid ink printing systems, for example ink jet printing systems, and, more particularly, to methods for improving resolution and print quality in four-color ink jet printing systems.
Known ink jet color printing systems use a three-color, for example, Cyan, Magenta and Yellow ("CMY") print head or "pen". All three colors are combined to form composite black (also called "process black") when black is needed. It is preferable, however, to print with (true) black rather than composite black for the following reasons:
1. Black looks better than composite black. Because composite black is composed of three colors, it often has a colored tint to it. It might appear, for example, as green-black, or blue-black. Also, the print quality of composite black varies with paper type, temperature and humidity.
2. In a typical computer system, print data is sent from the host computer to the printer to control the printing of each of the four colors, CMYK, where K represents black (to avoid confusion with the color blue). If an area on a page is printed with composite black, information must be sent to the printer for the CMY inks. If the same area is printed with a black pen (true black), only data for the K ink must be sent. So use of the black pen represents a potential three-to-one reduction in data transmission between the host and the printer.
3. Printing composite black requires the deposition of three times as much ink volume on the print media as is required for printing true black. This is not only more expensive to operate, but can also cause the media, especially plain paper, to develop a curl or develop other irregular perturbations in the area where the large volume of ink is deposited.
4. When printing composite black, the color pen must make three passes over the same region, the first pass putting down cyan ink, the second magenta and lastly yellow. If the same region is printed with the black pen, the black pen needs to make only one pass over the region to put down black ink. This represents a significant improvement in printing speed.
Some ink jet printers employ both a color pen and a black pen, but not for use at the same time. In other words, only one or the other pen may be used in the printer at one time. Commercial examples are the Hewlett-Packard DeskWriter C.TM. and DeskJet 500 C.TM.. Because only one pen can be used at a time, a page is either color or black, but not a mix of color and black on the same page. The user must manually swap pens before a page prints if the wrong pen is in the printer. When the color pen is in use, areas that should be black are printed using composite black. This compromises print quality and printing speed, for the reasons stated above.
Four-color printers are those having the three primary colors plus black (CMYK) available for use within a single page. Known four-color (CMYK) ink jet printers, however, have limited resolution and must print on special glossy paper. For example, the Hewlett-Packard PaintJet XL.TM. contains four print heads, one each for CMY and K. While that printer does not have to use composite black, its resolution is only 180 dots per inch (DPI). The Kodak DACONIX Color 4 Printer also contains four print heads for CMYK, only at 192 DPI. The Sharp JX-730 Color Ink Jet Printer is another four color printer that is 216 DPI. Both the Kodak and Sharp printers can print black adjacent to color, but again, at much lower resolution and only on special paper.
What is needed is a liquid ink printing system that allows mixing true black and color inks within a printed page and provides for high resolution printing on regular non-glossy paper. New inks are being developed for printing at high resolution, for example 300 DPI (dots per inch) on plain paper, i.e. a non-glossy paper such as bond paper used in a typewriter. Due to ink chemistry limitations, however, the color and black inks cannot touch on the page, and in fact cannot come within a minimum distance of each other on the page. When the inks do come within the minimum distance, the color ink draws the dye out of the black ink, causing the black ink to lighten. This lightening of the black ink on the page yields unacceptable print quality, and must be avoided. Accordingly, a need remains for maintaining a predetermined minimum spacing between black and color inks on a printed page.
One approach to maintaining a minimum spacing ("delta") required between color and black inks would be to examine each color dot on the page and then see if any of the surrounding dots within delta are black, and if they are, convert them to composite black. Such a method is not practical using present technology, for the following reasons. There are approximately eight million dots on an 8.5" by 11" page. For each dot, there are 138 surrounding dots within the minimum spacing--delta (see FIG. 1), each of which must be examined. Also, once a dot is converted from black to composite black, it is made of CMY (color) inks. Therefore, the algorithm must examine this new composite black dot's surrounding 138 dots to see if any are made of K ink. These procedures would easily require checking hundreds of millions or even billions of dots per page. The processing time would be excessive and, therefore, such an approach is not practical.
The processing problem is further complicated when the entire image is not available for evaluation at one time, such as when the image is generated in a piece-meal fashion. Generating the image piece-meal is commonly done in computers with a limited amount of memory to generate the image. By generating only a portion of the image at one time, known as banding, the computer needs only enough memory to manipulate one portion of image being generated, each of the portions being known as a band. After the computer has imaged a single band, it sends the band to the printer before beginning to form the next band of the image. In this way, the entire image can be generated in an amount of memory substantially less than required to generate the entire image at once, approximately 1/N the amount of memory, where N is equal to the number of bands used.
In many computer operating environments, e.g., Microsoft Windows, even if there exists sufficient memory in the computer to image the entire page, only a limited amount of memory is made available to image the page. In addition, the amount of memory available fluctuates, depending on the number of other programs currently consuming memory. By generating the image in bands, the banding process is able to dynamically adjust the size of the bands to match the amount of memory available in the computer.
The complication for the color separation process arises at the boundaries between adjacent bands. Because only a single band is imaged at a time, the banding process does not know the color properties of the pixels in an adjacent band. For example, if, in the current band, a vertical black line terminates on a band boundary, there is no information within the current band to determine whether there is color in the adjacent band within the minimum spacing of the black line. As a result, it cannot be determined whether to print the line in true black or composite black without some knowledge of future bands.